1. Field of the Invention
The present invention relates to a process and apparatus for dry cleaning a substrate by using photo-excited radicals.
2. Description of the Related Art
Recently, semiconductor devices such as LSIs have become highly integrated and the structures of the semiconductor devices complicated, and thus the methods of cleaning a semiconductor wafer must be remarkably improved.
Semiconductor wafers, etc. are wet cleaned by immersing the semiconductor wafers, etc. in a cleaning solution, a high purity acid or alkali solution or an ionized water, but such a wet cleaning does not provide a complete removal of contaminant metals such as iron (Fe) and sodium (Na) from the surface of a semiconductor wafer, etc. (a substrate to be cleaned) and it is impossible in a wet cleaning to avoid a recontamination of the substrate by a readherence of contaminants from the solution.
Accordingly, a dry etching process using a vaporization of contaminant materials by a gas is used as a cleaning process which provides an effective cleaning of even a complex device structure, and a complete removal of contaminants from the surface of a substrate. The present inventors proposed, as typical of such processes, a removal of contaminants by etching a thin surface layer of a silicon wafer with chlorine radicals (*Cl) formed by irradiation of a light to chlorine gas (Japanese Unexamined Patent Publication (Kokai) No. 62-42530). Also, the inventors proposed a process of bringing a thermally oxidized silicon layer into contact with chlorine radicals, to capture and remove contamination materials from the oxide layer without etching (Japanese Unexamined Patent Publication (Kokai) No. 1-217926).
These processes are characterized by a treatment by photo-excited chlorine radicals. FIG. 1 schematically illustrates a conventional apparatus for a dry cleaning, in which reference numeral 1 denotes a cleaning chamber made of quartz glass, 2 is a wafer or a substrate to be cleaned, 3 is a stage for the substrate, 4 is a transparent window made of a transparent quartz, 5 is light source (an ultra-violet source such as a high pressure mercury lamp), 6 is reflection mirror, 7 is a chlorine gas supply source, 8 is a flow meter, and 9 is an evacuation port.
In such an apparatus, a wafer 2 is heated to 200.degree.-400.degree. C. by a heater installed in the substrate stage 3, and a high purity (99.999%) chlorine gas is supplied to the reaction chamber 1 from the chlorine gas supply source 7 through the flow meter 8. The wafer 2 in the reaction chamber 1 is irradiated with an ultra-violet ray (wave length: about 200-300 nm) emitted from the light source 5 and reflected by the mirror 6. By making an energy of the ultra-violet ray about 20 mW/cm.sup.2, the chlorine gas in the reaction chamber 1 is activated to form radicals, and these radicals etch the wafer 2 so that contaminants are removed therefrom. The term "radicals" means neutral atoms of a reaction gas an outermost orbit of which is filled with electrons, and therefore, which are in the state having a very high reaction energy.
Nevertheless, the conditions for obtaining a maximum rate of a generation of radicals are not known, and therefore, an object of the present invention is to provide a photo-excited dry cleaning process and apparatus which enables an utilization of a high rate generation of radicals, to thereby improve the cleaning efficiency.
Also, the inventors found that, in a photo-excited dry cleaning process, an appropriate pressure of a reaction gas in a reaction chamber exists, and outside of this appropriate pressure, certain contaminants are not effectively removed and the surface of a substrate cleaned by the process is not flat or smooth, and that the appropriate pressure depends on the type of the substrate, contaminants, and reaction gas, etc. Therefore, another object of the present invention is to provide a photo-excited dry cleaning process at an appropriate reaction gas pressure which will effectively remove contaminants and obtain a flat and smooth cleaned surface, with a high radical generation or etching efficiency.